In recent years, a hybrid vehicle, which carries both of an electric motor and an engine and which is driven by either one or both of them, has been developed for practical use. Typical examples of hybrid vehicle include: a “series system” whose engine is used as a power source of an electric power generator and is only driven by a motor; a “parallel system” whose driving is mainly done by a motor at low speed and mainly done by an engine at high speed, wherein the motor drive assists the engine drive at start-up as well as at sudden acceleration; and a “series-parallel system” which is mainly driven by motor at both start-up and low speed, while distributing the engine drive and the motor at higher speed in a well-balanced manner. About these hybrid vehicles, engine is shut down during stoppage time; while when traveling only by motor drive or braking, operations such as controlling fuel system of the engine are carried out. Because of this, stoppage and operation of the engine are repeated. Consequently, it can be said that the engine oil used for these vehicles faces a specific usage environment compared with that of engine oil for conventional vehicle driven only by engine. However, such a lubricating oil for internal combustion engine specializing for hybrid vehicles has not been sufficiently studied; thereby, in fact, discovery thereof has been hardly obtained.
The present inventors had studied about characteristics of the lubricating oil suitably used for internal combustion engine of the above hybrid vehicles. As a result, when used in the hybrid vehicles, function of the metallic detergent tends to be deteriorated in a shorter time than that used in a conventional internal combustion engine; for the purpose of retention of engine performance and life extension of the lubricating oil, it is found out that high-temperature detergency of the new oil should be largely raised from the conventional level and the performance should be retained. When studied more specifically, about use conditions of the internal combustion engine for hybrid vehicles, particularly parallel system or series-parallel system hybrid vehicles, it is found that moisture mixed in the lubricating oil tends to be accumulated, due to the deterioration of the metallic detergent by hydrolysis, calcium carbonate tends to be agglomerated and base number of the lubricating oil also tends to be significantly decreased. Therefore, as a lubricating oil suitable for the internal combustion engine of the above described hybrid vehicle, an oil which essentially exhibits excellent hydrolytic stability and high performance of base number retention; further, for the purpose of retention of engine performance and life extension of the lubricating oil, the lubricating oil is required to have high-temperature detergency when being new oil, whose performance can be retained.
Normally, to the lubricating oil for internal combustion engine, zinc dithiophosphate is added as a combination of anti-wear agent and antioxidant; moreover, in order to enhance oxidation stability, high-temperature detergency, and acid-neutralizing ability, various additives such as overbased metallic detergent and ashless dispersant are blended. So as to minimize the impact on an exhaust emission control system and the like as much as possible, low-phosphorus and low-ash lubricating oil for internal combustion engine has been studied. Nevertheless, if the overbased metallic detergent is simply reduced for lowering ash content, high-temperature detergency and acid-neutralizing ability becomes insufficient. Hence, it is important to make the metallic detergent effectively function within the limited content and to retain the high-temperature detergency and acid-neutralizing ability at a high level for a long time. However, when low-ash oil in which zinc dithiophosphate is mainly contained is used, raising the level of high-temperature detergency is difficult. Especially, like the internal combustion engine of the hybrid vehicle, under the condition where moisture tends to be mixed and accumulated, it is quite difficult to make the overbased metallic detergent effectively function for a long time.
Recently, as a lubricating oil composition which shows excellent high-temperature detergency and long-drain performance of base number retention and so on, a sulfur-free lubricating oil (e.g. dialkyl zinc phosphate) and a lubricating oil containing sulfur-reduced phosphorus compound are proposed (for example, Patent Documents 1 and 2).    Patent Document 1: Japanese Patent Application Laid-open (JP-A) No. 2002-294271    Patent Document 2: Japanese Patent No. 3662228